This randomized, Phase I/III, multicenter, double-blinded, placebo-controlled study is designed to evaluate the safety and efficacy of atezolizumab (anti-programmed death-ligand 1 [PD-L1] antibody) in combination with carboplatin plus (+) etoposide compared with treatment with placebo + carboplatin + etoposide in chemotherapy-naive participants with ES-SCLC. Participants will be randomized in a 1:1 ratio to receive either atezolizumab + carboplatin + etoposide or placebo + carboplatin + etoposide on 21-day cycles for four cycles in the induction phase followed by maintenance with atezolizumab or placebo until progressive disease (PD) as assessed by the investigator using Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1). Treatment can be continued until persistent radiographic PD or symptomatic deterioration.